Electronic communication systems have become essential in the Information Age. Teleconferencing, mobile communications, and Internet technology have evolved from costly technological conveniences to necessary tools of modern communication and commerce. One communication tool increasingly exploited for its versatility and flexibility is the teleconferencing terminal. Teleconferencing has enjoyed widespread application in both personal and commercial communication contexts. The ability for several individuals at a Far-End location to participate in group discussions with several individuals at a Near-End location, has proved to be particularly valuable.
A speakerphone, which includes a speaker and microphone sections, is often integrated with a traditional telephone terminal to obviate the need for the traditional handset. The speaker and microphone of the speakerphone enable "hands free" operation, thereby making the telephone terminal available for use by all parties within range of the speaker and microphone. Due to the close proximity of the speaker and the microphone, it is necessary to employ circuitry for preventing the microphone from re-transmitting received audio signals produced by the speaker. Such re-transmission would otherwise cause undesirable echoes or sustained feedback oscillations. Some teleconferencing systems employ analog voice switching or echo suppression circuits. These circuits disable, or substantially attenuate, one of the respective send and receive telecommunication channels in favor of the other. One result of that type of echo suppression is undesirable clipping of voice signals during a conversation. Additionally, background noises are often completely muted during pauses in the conversation. Such muting can be undesirably perceived as disconnection of the call.
Digital adaptive echo cancellation technology has been developed as a favorable alternative to echo suppression. Adaptive echo cancellation techniques require digital signal analysis. The Far-End analog signal is converted to a digital signal, processed and then re-converted to an analog signal for output to the speaker. The microphone signal is similarly converted for digital processing. In the echo canceler, an estimated echo signal is produced by a digital filter and then subtracted from the microphone signal.
The filter used in the echo canceler to produce the estimated echo signal is an Adaptive Finite Impulse Response (AFIR) digital filter. The AFIR filter performs this function by convolving the received Far-End signal with internal coefficient values. The internal coefficient values are updated during a telephone conversation by an error correlation procedure, such that when the estimated echo signal is combined with the Near-End signal the echo signal is effectively canceled electronically. In order to ensure that the internal coefficient values are accurately updated, it is necessary to perform the error correlation procedure during periods of the conversation when only the Far-End participant is speaking. If the microphone signal contains a Near-End speech component, then the echo canceler will adapt the internal coefficients of the AFIR filter to attempt to cancel the Near-End speech as well as any reflected Far-End speech.
Known echo cancelers incorporate "double talk" detection circuits to identify conditions where both the Near-End and Far-End participants are speaking and to suspend AFIR coefficient adaptation when such conditions exist. The known detection circuits perform a comparison of the average energy of the loudspeaker signal to the average energy of the microphone signal. If the microphone signal level exceeds a predetermined proportion of loudspeaker signal, then adaptation within the digital filter is suspended. However, such double talk detection circuits are known to make incorrect determinations due to sharp changes in echo path response, changes in speaker volume, and the time varying properties of signals, among other factors.
In view of the state of art as described above, a double talk detection technique is desired which is capable of accurately distinguishing between a Near-End speech signal transmission and an echo signal, in order to effect echo cancellation or suppression more accurately and reliably.